In the field of internal combustion engine controls it is a well recognized principle that accurate ignition timing is necessary to maintain stable and efficient engine operation. In spark ignited engines, ignition occurs upon firing of the spark plug in the cylinder. The "timing" of such an engine is defined as the time at which an ignition signal is delivered to the spark plug relative to the time at which the piston reaches the end of its stroke in the compression cycle. This position is commonly referred to as "top dead center" (TDC), and timing is measured in crankshaft degrees with respect to TDC, for example 25 degrees before TDC (BTDC.)
In the past, ignition timing was controlled by magnetos and as such the timing angle could only be mechanically adjusted during servicing. Recently, electronics and in particular microprocessors have been adapted to control ignition timing. Electronic ignition controls have the advantage of being able to retard (closer to TDC) and advance (further from TDC) engine timing in response to sensed parameters, such as engine speed, engine load, air/fuel ratio and altitude. Generally, the electronic controls set a timing angle which is used for all engine cylinders; however, more sophisticated controls develop separate timing angles for each cylinder.
Capacitor Discharge Ignition Systems ("CDI's") typically include a charge storage mechanism, such as a capacitor, and a step-up transformer with a secondary coil connected to a spark ignition device, such as a spark plug. The ignition timing controller is adapted to discharge the capacitor through the transformer primary coil at the desired timing angle. Discharge of the capacitor through the transformer primary coil induces a high voltage signal in the transformer secondary coil, which, if sufficiently high, causes a spark to arc across the spark plug gap. The voltage applied across a spark plug must be greater than or equal to a predetermined characteristic "spark ionization potential" (voltage) V.sub.SP in order to initiate the spark. Such ionization potentials are typically on the order of 10 Kv or more. The ionization potential V.sub.SP is dependent on factors such as spark plug gap, cylinder pressure, engine load, and air/fuel ratio.
The time required for the transformer to energize to a level sufficient to cause sparking introduces a delay between the production of the ignition signal and actual ignition in the cylinder. This timing "error" can degrade overall engine operation and efficiency. To date, timing controls for engines having capacitor discharge ignitions have failed to address this problem. Partial compensation for the timing error could be achieved by introducing a predetermined timing offset. However, the delay between transformer energization and the spark plug firing is dependent on dynamic engine characteristics and performance factors, such as air/fuel ratio, cylinder pressure, and spark plug gap. As such a predetermined timing offset would only achieve marginal improvements at best.
The subject invention is directed toward overcoming the above problems by providing timing control which adjusts ignition timing to compensate for a delay between transformer energization and actual ignition.